Upgrading to Nexus 2.4 or later will provide a better solution though, since these per-repository threads no longer exist.

Note: Sometimes this problem can occur if another process on the system (not Nexus) is consuming so many process ID's that Nexus cannot spawn new threads. To check for this, first find the system wide limit.

You can do this with:

cat /proc/sys/kernel/pid_max

Then run this to find out the total number of threads/processes on the system: